What does error code E26 from TestCafe mean?

We are running TestCafe with numerous tests on a node.js web application with Chrome. From time to time we see error messages like this:

Error: { code: 'E26',
  isTestCafeError: true,
  callsite:
   CallsiteRecord {
     filename:
      'step_actions.js',
     lineNum: 101,
     callsiteFrameIdx: 6,
     stackFrames:
      [ [CallSite],
        [CallSite],
        [CallSite],
        [CallSite],
        [CallSite],
        [CallSite],
        [CallSite],
        [CallSite] ],
     isV8Frames: true } }

What does this error mean? I cannot find anything about it.

We're still trying to understand, what the reason for the error is. All I know is that it is only occurring when handling a pop up window.